U.S. Pat. No. 5,190,125 discloses a vehicle brake booster apparatus which has a reaction plate disposed between an output rod and a power piston. This apparatus also includes a valve mechanism which is arranged in the power piston. The valve mechanism controls the pressure differential between a constant pressure chamber and a variable pressure chamber and generates a control force of a master cylinder. The apparatus receives a reaction force from the master cylinder which is then transmitted to the power piston and the input rod in a predetermined ratio with a reaction mechanism (reaction disc).
The apparatus generates a correct brake pedal stroke in response to the piston stroke movement of the master cylinder. Further, the apparatus generates a reaction force of the brake pedal and a boosted force in response to the output of the reaction mechanism.
In brake boosters of this type, it is often desirable to improve the control response of the brake booster. One way of improving the control response is to design a large clearance and/or a large space in the control valve. However, in this apparatus, the reaction mechanism restrains the operation of the control valve and cannot produce a quick operation.
A need exists therefore for an improved brake booster apparatus that is able to overcome the above drawbacks.
It would be desirable to provide a brake booster apparatus which is able to produce a precise reaction force of the brake pedal in response to the amount pedal stroke operation.
It would also be desirable to provide a brake booster apparatus which can produce desirable output characteristics in response to the pedal stroke amount.
It would be further desirable to provide a brake booster apparatus possessing a large clearance between the atmospheric air and the negative pressure chamber.